Investigation of the influence of the sign of the g/g'-lattice misfit on the creep behaviour of single-crystal superalloys with g/g'-microstructure
Final Report Abstract
According to simulation results reported in the literature and obtained in this work, alloys with positive misfit should have a slightly higher creep resistance as compared to that of the alloys with negative misfit. Experimental verification of this prediction was the main aim of this project. It is shown that it is possible to change the sign of γ/γ’-misfit δ in Ni-base superalloys (from - to +) by lowering the Cr-concentration. Several Re-containing and Refree alloys with different δ-sign were proposed, solidified as [001] single-crystals and tested for creep at 800 °C and 1000 °C. The tests showed that, contrary to our expectations, the proposed alloy A4Cr with δ>0 have lower creep resistance as compared to the alloy A10Cr with δ<0: creep lifetime of the first alloy is about two times shorter than that of the second alloy. However, we assume that this result is probably not due to a change of the δ-sign but due to other side effects caused by lowered Cr concentration. In the present work, alloys with different δ-sign but similar solid-solution strengthening of the matrix, γ’-fractions and γ’-strengths were proposed. However, it was found that changing the δ-sign from - to + by lowering Cr also accelerates the γ’-coarsening. Cr, concentrating in the matrix, was found to have a similar stabilizing effect (but significantly less) on the γ/γ’- microstructure as Re. An additional negative effect is the bimodal γ’-size distribution in A4Cr and CM2.3(3.8)Cr, which also reduces the thermal stability of the γ/γ’-microstructure. These Cr depleted alloys have a higher γ’-solvus temperature, which makes their quenching more demanding. It should be mentioned that heat treatment of Ni-base superalloys with δ>0 is not a simple task. In contrast to Co-base alloys, here the γ’-precipitation occurs during cooling in the higher temperature range where δ changes its sign (from - to +) and, consequently, |δ| is very low. Therefore, if the cooling rate during quenching following the homogenization heat treatment is not high enough, a non-cuboidal γ’-phase is formed. Insufficiently deep homogenization also makes it difficult to obtain the cuboidal γ’-precipitates of equal size. Although Ni-base superalloys with superior creep resistance were not developed in this investigation, the authors of this work believe that designing Ni-base superalloys with δ>0 is promising. The development of such alloys has been performed under significant restrictions, which resembles “sack race”. The authors believe that without these restrictions one can develop a positive misfit Ni-base superalloy with acceptable creep strength. Here, it should be taken into account that fatigue strength and resistance to crack growth are significantly increased when P-rafting is formed. Therefore, alloys with δ>0 may be preferable for the turbine blades operating under strong cyclic loading.
Publications
- Creep of single-crystals of nickel-base -alloy at temperatures between 1150°C and 1288°C, Mater. Sci. Eng. A 825 (2021) 141880
A.I. Epishin, B. Fedelich, B. Viguier, S. Schriever, I.L. Svetlov, N.V. Petrushin, R. Saillard, A. Proieti, D. Poquillon, A. Chyrkin
(See online at https://doi.org/10.1016/j.msea.2021.141880) - Experimental determination of the temperature dependence of the structural-phase parameters of a nickel-base superalloy, Materialovedeniye (in Russian) issue 7 March (2021) pp. 3-12
I.A. Treninkov, N.V. Petrushin, A.I. Epishin, I.L. Svetlov, G. Nolze, E.S. Elutin
(See online at https://doi.org/10.31044/1684-579X-2021-0-7-3-12) - Investigation of elastic properties of the single-crystal nickel-base superalloy CMSX-4 in the temperature interval between room temperature and 1300 °C, Crystals 2021, 11(2), 152
A. Epishin, B. Fedelich, M. Finn, G. Künecke, B. Rehmer, G. Nolze, C. Leistner, N. Petrushin, I. Svetlov
(See online at https://doi.org/10.3390/cryst11020152) - Model for prediction of the temperature dependence of γ/γ’-lattice misfit in nickel-base superalloys, Materialovedeniye (in Russian) issue 3 March (2021) pp. 9-19
A.I. Epishin, N.V. Petrushin, I.L. Svetlov, G. Nolze
(See online at https://doi.org/10.31044/1684-579X-2021-0-3-9-18)
